Simplify Away Refutation Stage
Simplify away killer stage to a constant bonus given to the killer move during
quiet move scoring.
Passed Non-regression STC (Against then-pending PR #5472):
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 106176 W: 27685 L: 27539 D: 50952
Ptnml(0-2): 410, 12765, 26637, 12821, 455
https://tests.stockfishchess.org/tests/view/668dd0835034141ae5999e8f
Passed Non-regression LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 92472 W: 23426 L: 23276 D: 45770
Ptnml(0-2): 55, 10376, 25215, 10544, 46
https://tests.stockfishchess.org/tests/view/669019e45034141ae5999fd2
closes https://github.com/official-stockfish/Stockfish/pull/5476
Bench 1459677